Background:  We enrolled 40 HIV-infected subjects with CD4⁺ T-cell counts of > 350 cells/mL and plasma HIV RNA levels of < 50 copies/mL while receiving ART in a phase I, randomized, double-blind, placebo-controlled dose escalation study of EP HIV-1090, a multi-epitope DNA vaccine that encodes 21 HIV-1 CTL epitopes, and PADRE, a potent HTL epitope. Subjects were randomized 4:1 to receive either EP HIV-1090 or placebo in 4 intramuscular doses of 0.5, 1, 2, or 4 mg of vaccine or placebo at 0, 4, 8, and 16 weeks.

Methods:  Primary vaccine immunogenicity was measured to individual vaccine epitopes in an interferon (IFN)-g ELISpot assay using peripheral blood mononuclear cells (PBMC) obtained at baseline and at week 18, 2 weeks after the last immunization. In a secondary analysis, we evaluated epitope-specific IFN-g responses by ELISpot following in vitro stimulation of baseline vs week 24 PBMC with pooled epitope peptides. A vaccine responder was defined prospectively as having ELISpot responses following vaccination to ≥ 2 epitope peptides that were ≥ 3 times those of pre-vaccine values.

Results:  Baseline ELISpot analyses indicated that 9 of the 40 subjects responded to 1 or more vaccine epitopes, and 9 of 21 peptides were recognized by at least 1 subject. Post-vaccination, 11 of 21 peptides were recognized by 11 subjects. Two subjects (0.5- and 1-mg dose groups) met predetermined criteria for a vaccine response (2 of 32 [6.3%] of subjects receiving vaccine) based on ELISpot, and an additional 3 subjects developed vaccine responses to 1 vaccine epitope (5 of 32, 15.6%). Using the in vitro stimulation assay, 9 of 32 (28.1%) vaccine recipients developed responses to 2 or more vaccine epitope peptides. Overall, the frequency of vaccine recipients who developed a response to 1 or more vaccine epitopes using both types of assays was 17 of 32 (53.1%), although this was not significantly greater than the placebo response rate (3 of 8, p = 0.69). Tetramer and intracellular cytokine analyses are ongoing.

Conclusions:  These results suggest that priming of new T-cell responses without major in vivo expansion of vaccine-specific IFN-g-producing T cells may have occurred in some subjects receiving this multi-epitope DNA vaccine. Future vaccine studies using analytical treatment interruptions might better determine a role for DNA-primed CD8⁺ T-cell responses targeting highly conserved HIV-1 epitopes in suppressing HIV-1 replication in vivo.

Keywords: DNA vaccine; immunogenicity; epitopes